1. Field of the Invention
The present invention relates to a piston type variable displacement compressor. More specifically, this invention relates to a piston type variable displacement compressor which adjusts the pressure in a crank chamber to control the inclined angle of a swash plate based on the difference between the pressure in the crank chamber and the suction pressure.
2. Description of the Related Art
In general, compressors are mounted in vehicles to supply compressed refrigerant gas to the vehicle's air conditioning system. To maintain the air temperature inside the vehicle at a level comfortable for the vehicle's passengers, it is important to use a compressor whose displacement is controllable. One known compressor of this type controls the inclined angle of a swash plate, tiltably supported on a drive shaft, based on the difference between the pressure in a crank chamber and the suction pressure, and converts the rotational motion of the swash plate to reciprocal linear motion of each piston.
A conventional piston type compressor disclosed in U.S. Pat. No. 5,173,032 uses no electromagnetic clutch for the transmission and blocking of power between an external driving source and the drive shaft of the compressor. The external driving source is coupled directly to the drive shaft.
The clutchless structure with the driving source coupled directly to the drive shaft can eliminate shocks which would otherwise be produced by the ON/OFF action of such a clutch. When such a compressor is mounted in a vehicle, passenger comfort is improved. The clutchless structure can also reduce the overall weight of the cooling system and thus reduce costs.
In such a clutchless system, the compressor runs even when no cooling is needed. With such compressors, it is important that when cooling is unnecessary, the discharge displacement be reduced as much as possible to prevent the evaporator from frosting. When no cooling is needed or there is a probability of frosting, the circulation of the refrigerant gas through the compressor and its external refrigeration circuit should be stopped. The compressor described in the aforementioned U.S. patent is designed to block the flow of gas into the suction chamber from the external refrigeration circuit by the use of an electromagnetic valve to stop the circulation of the refrigerant gas.
In the compressor described above, when the flow of the gas from the external refrigeration circuit into the suction chamber is blocked, the pressure in the suction chamber drops drastically and the control valve responsive to that pressure opens fully. The full opening of the control valve allows the gas in the discharge chamber to flow into the crank chamber, which in turn raises the pressure in the crank chamber. When the pressure in the suction chamber falls, the suction pressure in the cylinder bores falls, too, thus increasing the difference between the pressure in the crank chamber and the pressure in the cylinder bores. This pressure differential in turn minimizes the inclination of the swash plate which reciprocates the pistons. As a result, the discharge displacement is minimized. At this time, the driving torque needed by the compressor is minimized, thus reducing power loss as much as possible.
The crank chamber is communicated with the suction chamber by a through hole. When the flow of gas from the external refrigeration circuit to the suction chamber is blocked, the gas that is discharged into the discharge chamber from the cylinder bores is drawn into the crank chamber by way of the opened control valve. The gas in the crank chamber flows into the suction chamber by way of the through holes. The gas is then drawn into the cylinder bores during the suction stroke of the piston. In other words, when the flow of gas from the external circuit is blocked, the cylinder bores, discharge chamber, crank chamber, suction chamber, and cylinder bores establish a gas circulation path in the compressor. A lubricating oil is suspended in the gas. The lubricating oil is conveyed in the circulation path together with the gas during circulation of the gas. The lubricating oil lubricates the parts inside the compressor.
When the gas flow to the suction chamber from the external refrigeration circuit is commenced, the pressure in the suction chamber rises, and then the control valve closes. This inhibits the gas flow into the crank chamber from the discharge chamber, lowering the pressure in the crank chamber. As the pressure in the suction chamber rises, the suction pressure in the cylinder bores rises too. The difference between the pressure in the crank chamber and the pressure in the cylinder bores therefore becomes smaller, and the inclined angle of the swash plate is increased to it's maximum, maximizing the discharge displacement.
To reduce power loss, the inclined angle of the swash plate should be minimized as much as possible when the flow of gas into the suction chamber is inhibited, or when the discharge displacement is minimized. However, the minimum swash plate angle must be determined while considering lubrication of the compressor.
The gas discharged into the external circuit by the compressor returns to the compressor after performing heat exchange with a condenser and an evaporator provided in the external circuit. Lubricating oil in the compressor is conveyed to the external circuit suspended in the gas and returns to the compressor together with the gas. However, the gas flowing in the external circuit must be more than a predetermined amount to return the lubricating oil to the compressor together with the gas. The flow amount of the gas depends on the inclined angle of the swash plate. Therefore, when the inclination of the swash plate is too small, the gas flow is smaller than the predetermined amount. This results in only the gas returning to the compressor from the external circuit. Hence, when the gas in the compressor is discharged, with the lubricating oil suspended therein, into the external circuit and returned to the compressor without the lubricating oil, lubrication within the compressor will be insufficient.
When the flow of gas from the external circuit to the suction chamber is blocked, lubricating oil does not flow from the inside to the outside of the compressor since the gas inside the compressor circulates in the circulation path while the gas outside the compressor remains outside. However, when flow of gas from the external circuit to the suction chamber is commenced, the compressor will be inadequately lubricated since the flow of gas is below an amount required to return the lubricating oil into the compressor from the external circuit. Therefore, with the compressor disclosed in the above U.S. patent, it is necessary that the gas flow rate in the external circuit be more than that required to return the lubricating oil into the compressor when inclination of the swash plate is slightly larger than its minimum inclined angle. In other words, the minimum inclined angle of the swash plate must be larger than that enabling a sufficient flow of gas to return the lubricating oil from the outside of the compressor to its inside.